The continued growth in wireless communications is demanding personal base stations, portable handsets and other communication terminals that are compact, light and able to perform a variety of functions. Considerable size reductions have already been achieved through the integration and miniaturization of most of the electronic and radio frequency (RF) circuitry in the communication terminal. However, the conventional antennas typically used remain unduly large relative to the terminal. This is particularly true for designs which utilize multiple antennas in order to provide diversity, interference reduction and beamforming. A conventional antenna with a low profile structure suitable for mounting on personal base stations, portable handsets and other communication terminals is known as the planar inverted-F antenna (PIFA).
FIG. 1 illustrates an exemplary PIFA 10 in accordance with the prior art. The PIFA 10 includes a ground plane 12, an L.sub.p .times.W.sub.p rectangular radiating patch 14 and a short-circuit plate 16 having a width d.sub.1 which is narrower than the width W.sub.p of the radiating patch 14. The short-circuit plate 16 shorts radiating patch 14 to the ground plane 12 along a null of the TM.sub.100 dominant mode electric field of patch 14. The PIFA 10 may thus be considered a rectangular microstrip antenna in which the length of the rectangular radiating patch 14 is reduced in half by the connection of the short-circuit plate 16 at the TM.sub.100 dominant mode null. The short-circuit plate 16 supports the radiating patch 14 at a distance d.sub.2 above the ground plane 12. The radiating patch 14 is fed by a TEM transmission line 18 from the back of the ground plane 12, at a point located a distance d.sub.3 from the short-circuit plate 16. The transmission line 18 has a width d.sub.4 and includes an inner conductor 20 surrounded by an outer conductor 22. A detailed analysis of the operation of the conventional PIFA 10 of FIG. 1 may be found in K. Hirasawa and M. Haneishi, "Analysis, Design and Measurement of Small and Low-Profile Antennas," Artech House, Norwood, Mass., 1992, Ch. 5, pp. 161-180, which is incorporated by reference herein. The PIFA 10 is particularly well-suited for use in personal base stations, handsets and other wireless communication terminals because it has a low profile, a large bandwidth and provides substantially uniform coverage, and because it can be implemented using an air dielectric as shown in FIG. 1. The bandwidth of the PIFA 10 may be further increased by using a conducting chassis of a terminal housing as the ground plane 12. This is due to the fact that the radiating patch 14 will then have a size comparable to the ground plane and will therefore induce surface current on the ground plane.
A significant problem with antennas such as the conventional PIFA 10 of FIG. 1 is that the radiating patch is fed by the TEM transmission line 18 or a similar structure such as a coaxial line. This generally makes the PIFA more difficult to manufacture, in that the relative position and other characteristics of the feed must be implemented with a high degree of accuracy, and the outer and center conductors must be properly connected. Moreover, the cost of a TEM transmission line or coaxial line and its associated connector is excessive, and may be several times the cost of the rest of the antenna. In addition, the use of a TEM transmission line or a coaxial line limits the tuning flexibility of the antenna feed in that the characteristics of such lines are not easily adjusted during or after manufacture. A TEM transmission line or a coaxial line may also be relatively difficult to interconnect with related circuitry in a personal base station, portable handset or other communication terminal. These and other factors associated with the use of a TEM transmission line or coaxial line feed unduly increase the cost of the antenna, and prevent its use in many cost-sensitive applications. It would therefore be desirable if an alternative feed mechanism could be developed such that the low profile, large bandwidth and uniform coverage advantages of PIFAs could be provided in personal base stations, handsets and other communication terminals without the drawbacks associated with transmission line feeds such as that shown in FIG. 1.
As is apparent from the above, a need exists for an improved PIFA which avoids the excessive cost of conventional transmission line or coaxial feeds, is simpler to manufacture and integrate with related terminal circuitry, and provides more tuning flexibility, without sacrificing the low profile, large bandwidth and uniform coverage advantages typically associated with PIFAs.